A common type of flush valve for flushing water from a tank or water closet to a toilet bowl, includes a valve member pivotally mounted on a valve seat, the valve member including a hollow tank ball. When the valve member is pivoted up a short distance from the seat, the large buoyancy of the tank ball causes it to "pop" open to allow water to rapidly pass out of the water closet. When the tank ball pivots up, it partially fills with water so its buoyancy is reduced, although it still has positive buoyancy. As water in the water closet drops to the level of the open tank ball, the tank ball floats near the water surface and starts to pivot down as the water level falls. As the tank ball approaches the valve seat through which water is rapidly flowing, suction causes the tank ball to suddenly move down and close the flush valve, while a few inches of water remain in the water closet above the level of the valve seat. Older water closets typically had capacities of 5 to 7 gallons which was more than adequate to fully flush a toilet bowl. The retention of a few inches of tank water in such water closets did not noticeably affect flushing but had the advantage of saving on water usage.
Many modern toilets have water closet of reduced capacity, to hold perhaps 31/2 gallons of water, to reduce the required height of the water closet to save on water usage, and sometimes also for aesthetic purposes. The economical and reliable prior flush valves which include a hollow tank ball on the valve member, are commonly installed in such toilets. Although the toilets often function well, there are many cases in which flushing is not complete because of insufficient flushing water passing through the toilet bowl in each flushing cycle. This results in double flushing, which results in wasting considerable water in an installation that may have been made to save water. Whether the toilet will flush well depends upon many factors such as whether and at what angle the flush valve seat is angled from the vertical, and by what angle the valve seat pivots to its open position. It would be desirable if an economical and reliable prior type flush valve could be used in modern toilets having water closets of reduced capacity, and yet if such installed valves could be modified rapidly and at low cost without disassembling them, when required to increase the amount of flush water.